There are a large number of pointing devices for inputting positional data to a computer. The most commonly used of these is the “mouse”. The mechanical version of the mouse consists of a handheld object that is moved over a surface. A ball within the device is in contact with the surface and rolls as the device is moved. The movement of the ball is sensed by detectors in the device which report the amount of motion along each of two directions periodically to the computer. The mechanical mouse picks up oil and other substances from the surface over which it rolls. These contaminants build up on the rollers used to sense the ball direction and interfere with the free motion of these rollers unless the rollers are periodically cleaned. The need for such cleaning operations has led to the introduction of the optical mouse, which does not suffer from this problem.
The optical mouse illuminates the surface over which the device is moved and periodically forms images of the surface. The illumination system enhances the rough features of the surface. The images are taken at time intervals that guarantee that two successive images share a significant portion of the field of view. By comparing the images, the distance and direction the device moved between the images can be determined.
While the mouse is an excellent solution to the problem of controlling the motion of a cursor on the computer screen, it is poorly suited for simulating a pen that can be used to input handwriting and the like. A number of pen-like pointing devices have been introduced to fill this need. One class of pen devices uses a stylus or similarly shaped object that is held like a pen such that its end moves over a special surface. The position of the pen on the surface is detected by sensors associated with the surface, and the position data is sent to the data processing system attached to the device. While such devices more nearly emulate a pen, they require a special writing surface that includes the sensors needed to track the stylus's motion. The cost of such systems is much greater than the cost of an optical mouse, and hence, these systems have not found wide spread acceptance. In addition, the tablets on which the user “writes” are of a fixed size. If the tablet is large, it is not suitable for portable computers. Similarly, if the tablet is small, the amount of data that can be written is significantly reduced.
An optical mouse, implemented in a pen-like housing, in principle, overcomes these problems. In such a device, an illumination system illuminates the surface of the workspace just under the pen, and an imaging system forms successive images of the surface as the pen moves over the surface. The successive images are then compared to one another to determine the amount and direction of motion of the pen between the images. Unfortunately, when a user “writes” with the device, the user tends to rotate the device in the user's hand as the user moves the device over the underlying surface. This rotation complicates the image comparisons, since the images must be now be compared for various displacements and rotations.